1. Field of the Invention
The present invention relates generally to semiconductor wafer cleaning, and more particularly, to methods, systems and apparatus for cleaning wafer edges before, during and after fabrication operations.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well known that there is a need to clean a semiconductor substrate (i.e., wafer) where a fabrication operation has been performed that leaves unwanted residuals on the surface of the wafer. Examples of such a fabrication operation include plasma etching, material depositions and chemical mechanical planarization (CMP). CMP is commonly performed on both dielectric materials and conductive materials, e.g., such as oxide and copper. If particles or films are left on the surface of the wafer for subsequent fabrication operations, the unwanted residual particles or material may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features or with subsequent lithography operations. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is therefore necessary to clean the wafer adequately yet efficiently after fabrication operations that leave unwanted residue on the surface of the wafer.
FIG. 1A shows a high level schematic diagram of a wafer cleaning system 50. The cleaning system 50 typically includes a load station 10 where a plurality of wafers in a cassette 14 may be inserted for cleaning through the system. Once the wafers are inserted into the load station 10, a wafer 12 may be taken from the cassette 14 and moved into a brush box one 16a, where the wafer 12 is scrubbed with selected chemicals and water (e.g., de-ionized (DI) water). The wafer 12 is then moved to a brush box two 16b. After the wafer has been scrubbed in the brush boxes 16, the wafer is moved into a spin, rinse, and dry (SRD) station 20 where DI water is sprayed onto the surface of the wafer and spun to dry. During the rinsing operation in the SRD station. After the wafer has been placed through the SRD station 20, the wafer is moved to an unload station 22.
FIG. 1B shows a simplified view of a cleaning process performed in brush box one 16a. In brush box one 16a, the wafer 12 having a top surface 12a (i.e., the active side) is inserted between a top brush 30a and a bottom brush 30b. The wafer 12 is capable of being rotated by holding and driving rollers (not shown) and the rotating brushes 30a and 30b to adequately clean the entire top and bottom surfaces of the wafer. After typical CMP operations, a wafer is placed into the cleaning station 50. In brush box one 16a, the top brush 30a and the bottom brush 30b are preferably concentrated with a cleaning chemical, which is received from a source 32 or other sources controlled by a chemical/DI water dispensing system (not shown).
A common fabrication operation includes the deposition of metals over previously formed dielectric features, which is commonly done in damascene and dual-damascene processes. As is generally defined, damascene and dual-damascene processes include the formation of features, such as interconnect lines and vias into dielectric materials, filling the dielectric features with conductive material, e.g., such as copper, and then performing CMP operations to remove the excess metallization material. The metal material can be formed over the wafer using various techniques, such as, for example, deposition, electroplating, sputtering, and the like.
In either case, the formation of metal material may generate excess beading around the periphery of the wafer. It is also a common operation to perform standard cleaning operations after such metal deposition operations, to ensure that the excess metal and loose particles and contaminants are removed from the wafer before engaging in further processing.
A problem typically experienced is that standard brush scrubbing and edge cleaning techniques fail to clean and remove the metal edge beading and loose particles from wafer edge surfaces sufficiently well from the upper portion of the wafer surface in the edge exclusion zone between 1 and 3 millimeters from the wafer bevel edge. One approach to edge cleaning was described in U.S. Pat. No. 5,861,066 entitled “Method and Apparatus for Cleaning Edges of Contaminated Substrate.” This U.S. Patent is incorporated herein by reference. Although this apparatus does well at cleaning the immediate edge of the wafer, other portions of the wafer edge in which beading and particulates adhere are most commonly not sufficiently addressed. That is, although sufficient center cleaning is performed using the brushes 30 of FIG. 1B, not enough mechanical scrubbing is performed directly on the top and bottom surface areas near the edge. Consequently, residue will remain even after repeated conventional brush cleaning.
FIG. 1C shows another prior art wafer brush-box 60 with a double-sided wafer edge cleaner 65. The brush-box 60 includes a drive roller 61 that rotates in a direction 62 that drives the wafer 12 in a direction 63, and a stator roller 68 that maintaines the wafer contact with the drive roller 61. The edge cleaner 65 cleans the edge zones 66 of both the front side and the back side of the wafer 12 and the bevel edge 74 of the wafer 12, typically rotating at a different speed than the drive roller 61. The edge cleaner 65 can be a stationary, U-shaped scrub brush 70 as shown in detail in FIG. 1D. The stationary, U-shaped scrub brush 70 includes a compressible scrubbing pad 72 that the bevel edge 74 of the wafer 12 and the front side and back side of the wafer 12 compresses as the wafer 12 is pressed into the scrubbing pad 72. The edge cleaner 65 can also be a rotating, grooved edge cleaning roller 80 as shown in detail in FIG. 1E. If the edge cleaner 65 is a rotating, grooved edge cleaning roller 80 can rotate in a direction 67 as shown in FIG. 1C. The grooved edge cleaning roller 80 includes a compressible scrubbing pad 82 that the wafer 12 compresses as the wafer is pressed into the scrubbing pad 82.
Unfortunately, the double-sided wafer edge cleaner 65 has several shortfalls. First, the double-sided wafer edge cleaner 65 cannot provide a uniform edge cleaning process as shown in FIG. 1F. The inner edge of the cleaned zone is shown by line 90. The cleaning edge 90 is erratic, in part, because the compressible scrubbing pad 72, 82 compresses and rebounds (i.e., extends) as the double-sided wafer edge cleaner 65 passes along the bevel edge 74 of the wafer 12.
Another problem with the double-sided wafer edge cleaner 65 is that the scrubbing pad 72, 82 cannot be cleaned or rinsed very well due to the deep grooved shape and the compressed nature of the scrubbing pad 72, 82. As a result contaminants and debris can become lodged deep within the scrubbing pad 72, 82 and contaminants can be transferred from one wafer to the next wafer and debris removed from one wafer can damage a subsequent wafer.
The double-sided wafer edge cleaner 65 also does not provide for independent control of front side and back side edge exclusion cleaning so that differential cleaning on the front side and back side of the wafer can occur, i.e., the double-sided wafer edge cleaner 65 does not allow independent control of the cleaning width on the front side and the back side of the wafer 12. The double-sided wafer edge cleaner 65 can also pinch or bind the wafer 12 in the deep pocket of the scrubbing pad 72, 82. Binding can damage the wafer 12 or result in less effective cleaning of the bevel edge 74 and the edge zone 66 of the wafer 12. Further, systematic wear of the scrubbing pad will change the cleaning performance and effectiveness of the scrubbing pad over time, reducing the bevel edge contact characteristics or the front and back side edge exclusion cleaning.
In view of the foregoing, there is a need for a double sided substrate edge cleaning system, method and apparatus that provides an improved substrate edge cleaning mechanism that provides improved control of front side and back side edge cleaning, and that does not bind the substrate being cleaned.